Compensation amplifier system



M. ARTZT COMPENSATION AMPLIFIER SYSTEM Filed Jne's. 1938 4 sheets-sheet 1 ma YN NMS I INVENTOR.

MlaR/CE ,4A7'Z7' I ATTORNEY.

Nm w M I s bm. MM. Q m w+ mm L/.Tw n mi New mm I.. mm m o Y w SES :S

April 3o, 1940. E M, ARTZT 2,199,192

COMPENSATION AMPLIFIER SYSTEM Filed Jvune 8, 1938 4 Sheets-Sheet 2 April 3o, 1940. M. AR-rzr 2,199,192

COMPENSATION AMELIFIER, SYSTEMv Filed June 8, 1958 4 Sheets-Sheet 5 I NV EN TOR.

A TTORN EY.

MAUR/CE RTZT April 3o, 1940. M, ARTZT 2,199,192

COMPENSATION AMPLIFIER SYSTEM 'MAUR/CE ARTZ' A TTORNE Y.

Patented Apr. 30, 1940 UNITED STATE COMPENSATION AMPLIFIER SYSTEM:

Maurice Artzt, Haddonfeld, N. J., assignor to v Radio Corporation of America, a corporation of Delaware Application June 8, 1938, Serial No. -212,433

10 Claims.

This invention relates to an amplifier system wherein it is possible to compensatefor undesirable conditions in amplification because of non-linear frequency response or because of nonlinear response devices in order that the output from the amplifier may be linear. The ysystem also affords a means whereby peculiarities in the response of an electrical translating apparatus may be compensated for to `produce a proper responsel at the electrical apparatus.

More specifically, lthe present system is applicable to facsimile recording systems and may be used to amplify signals of variable frequency and amplitude as received yfrom a facsimile scanner with means to compensate for any deciencies in strength of certain of these signals in order .that the recording reproduced at the ,facsimile receiver may be amore faithful reproduction of the original subject being transmitted.

In facsimile systems it is seldom possible to have all of the parts and elements of the system operate in sucha manner as to maintain strict linear amplification and response characteristics. In the recording of true half-tones certain errors 'are generally present, which,` if not compensated for, would produce an undesirable, if not illegf ible, recording, and to compensate for suchy errors it is desirable to introduce a corrective or compensation device which may be used to'maintain the signals as utilized at the facsimile receiver or translating means at a level which is linear with respect to the light values of the subject being transmitted. It is also desirable that this compensation be variable and controllable so that as various parts of the system are improved the degree of lcorrection can be changed to retain over all straight line amplification characteristics. Furthermore, another reason for 'maintainingthe compensation amplifier' system adjustable and variable is to provide a means `whereby inherent changes in the response of the system due to usey and deterioration which would, without the compensation system, produce errors, may be compensated for by changing the adjustments of the compensation amplif'ler.

Furthermore, in facsimile systems particularly where a receiver employing the carbon process is utilized such as is shown and described in the patent to Charles J. Young, Reissue #20,152 of October 27, 1936, appropriately responsive means must be introduced in the system to provide a .compensation for the non-linear pressure response of the carbon paper used in the receiver. In such an instance, it has been found that a true linearl amplication' is not dition, because, the amount of carbon which is the best contransferred to the recording paper as a function of the pressure of the printer bar does not vary in signal amplitude, particularly in the low siglinearly. rIt has been found that small changes nal levels, do not have the proper effect and are not properly responded to in the carbon paper facsimiley process. 4 to provide means whereby the amplitude c-haracteristics of the amplifier increase rapidly with small changes in input at the low-signal level, and gradually change to a lower and lower percentage change in'output as the signal level increases. When the proper compensation is provided true half-tonesybecome possible' with a carbon paper recorder. When a compensation amplifier system is utilized for the non-linear response of a carbon paper recorder, it is also desirable to have the-system adjustable and variable since not all carbon papers respond in the same manner.- Accordingly, when a different type of carbon paper is used, or in fact, when dierent rolls of the same type are used, it may be necessary to adjust the compensation amplifier to maintain proper half-tone recordings in viewof changes in the pressure response of the carbon paper being utilized at the particular instant.

' This invention therefore relates to a compensation amplifier system in which the slope of the kcorrecting curve may be changed at different pre-set-points and may further be changed to have either a positive or negative slope in order that substantially any and all desirable vcompensation fac-tors may be introduced in the amplifier system to .compensate for the nonlinear response of one or more parts of the entire transmitting and receiving system.

` It is therefore yone purpose ofthe present in` vention to provide an vamplifier systernwherein an apparatus having a non-linear response may 'be so controlled as to produce amplified signals Awhich are linear with respect to the originating variations. Another advantage of the present systemre sides in the provision of means whereby various compensations may be introduced in an amplifier system to provide either linear or non-linear amplication response as may be desired under particular circumstances. Still another advantage of the present inventionY resides in the provision of an amplifier systemrwherein the non-linear response of an electricaltranslating or receiving apparatus may be As a result, `it is desirable compensated for'by introducing distortions in l55 the amplifying system whereby the electrical translating or receiving apparatus, in responding to the distorted signals, may be operated with an over-all true linearity.

A still further purpose of the present invention resides in the provision of means whereby variations in the pressure response of a carbon. paper facsimile receiver may be compensated for to permit the true reproduction of half-tones.

Still another advantage of the present invention resides in the provision of means whereby the slope of the correcting curve of the amplifier may be changed and whereby the slope may be set for either positive or negative compensation.

Still another advantage of the present invention resides in the provision of means whereby such compensations may be applied to a modulated signal without the necessityl of demodulating the signal, correcting the signal and again re-modulating.

Still other advantages and purposes of the present invention will become more apparent to those skilled in the art from a reading of the following specication and claims, particularly when considered with the drawings, wherein like gures represent like parts, and wherein:

Figure l. illustrates one form of the present invention, wherein the compensation is introduced after de-modulation of the input signal.

Figure 2 illustrates a modication of Figure 1 and another form oi the present invention.

Figure 3 shows still another form of the invention wherein compensations may be introduced without the necessity of de-moolulating the input signal.

Figure 4 illustrates various curves showing the possible over-all compensation response of the system shown in Figure 1.

Figure 5 shows a possible response curve of the system shown in Figure 2.

Figures 6 and 7 illustrate various response curves of the system shown in Figure 3.

Referring now to Figure 1, wherein one form of the present invention is illustrated schematically, the input signal is applied to the primary of transformer l2. This input signal is normally in the form of a modulated tone signal, since ordinarily some scanning tone frequency is 'applied to the scanning head of a facsimile transmitter. This tone signal is rectified by the discharge tube ld, which comprises a cathode and a pair of anodes, and which operates as a i'ull wave rectier. The rectified signal is then passed through the filter network which comprises a choke l5 and condensers it. After the rectied signal has been iiltered it represents the envelope of the modulated tone which is applied to the primary transformer i2. rThe ltered voltage is then applied to a second rectifier tube I 8 comprising a cathode and a pair of anodes i9 and 2U. This rectifier tube will be referred to as a threshold rectier and its specific operation will be explained later. The anode 23 is connected to the control electrode of a discharge tube 22, the control electrode being connected to ground by the grid resistor 23, while the anode I9 is connected to the control electrode of discharge tube 25. The control electrode of this tube is connected to ground by way of resistances 26 and 2l.

For operating the discharge devices a source of potential (not shown) is provided, and is connected to terminals 3l and 32, the ylatter of which is connected to ground. A series of `resistances 2l, 28, 2Q and 35 are connected between positive terminal' 3l and groundjn order to operate as a potential divider from which different degrees of positive potential may be made available.

The anode of discharge tube 25 is connected to a point between resistances 29 and 35 by a pair of parallel resistances 34 and 35, each of which operate as potentiometers and are each provided with an adjustable contact 36 and 3l respectively. The anode of discharge tube 22 is connected to the adjustable contact 3E of resistance 34, so that the portion R of the resistance 34 is common to the anode circuits of both tubes 22 and 25.

For the purpose of again modulating the signal, a pair of tubes 33 and 46 are provided, and to provide a source of carrier frequency an oscillator tube 42 is also provided. The tube 42 and its immediately associated circuit comprises a standard oscillator, the oscillations being present in the primary of transformer 43. The secondary of the transformer 43 is connected in pushpull to the rst control electrodes of discharge tubes 39 and 4U. The anodes of discharge tubes 39 and 4D are connected to the end terminals of the primary of the output transformer 44, the secondary of which is connected to the output terminals 45 and 46. 'I'he screen or #2 grids of the discharge tubes 39 and 4t2 are connected together and are in turn connected to the source of positive potential and to the midpoint of the primary winding of transformer-44. The suppressor or #3 grids of discharge tubes 39 and 40 are connected together and are in turn connected to the adjustable point 3l of the resistance 35. When the carrier frequency, which originates at the oscillator 42, is applied to the control electrodes (#1 grids) of the discharge tubes 39 and 45, an output will be present in the secondary of transformer 44 of amplified carrier frequency. The intensity of the carrier frequency in the output depends upon the potential of the #3 grids, this potential being determined by the voltage drop across that portion of the resistance 35 between the contact 31 and the point between resistances 29 and 30. Under a condition of zero input signal, the discharge tubes 22 and 25 dra-w sucient current through their common plate resistor that the modulator discharge tubes 39 and 4E) are biased to cut-off. The output from the rectier I4 is applied between the cathode of rectifier I8 and an adjustable contact 33 along resistance 28.

To simplify the explanation of the operation of the system, the voltage drop which appears across the resistance 2l will be designated as the voltage e, While the voltage dropy across that portion of the resistance 28 between the adjustable contact 33 and the cathode of discharge tube 25 will be designated as the voltage e. It will be noticed therefore that the potential of the anode i9 of tube i8 is negative with respect to its cathode by an amount equal to the voltage e. Furthermore, the anode 25 is negative with respect to the cathode by an amount equal to the sum of voltages e and e. Therefore, the rectiiied signal, as it appears at the output of the filter circuit, must rise above the Value e before the anode IS of tube i8 will be supplied with any current. At this signal level, andl above, there will be anode current flowing through the resistance 25. 'I'he presence of the current flowing through resistance 26 produces a potential drop which drives the control electrode of discharge tube 25 in a negative direction, tending to reduce it to cut-off. This reduces the amount of anode ciu'- voutput remain substantially constant. 'This is `particularly'advantageous since no adjustments `other than the setting of the adjustable contact rent which `liows 'through tube 25 and'accord# ingly reduces the' negative biasv of the modulator tubes 39 and 40, permittingcarrier frequency to be transferred therethrough by an amount determined by the level of the rectified voltage applied to the cathode of tube I8 and thecontact 33 level which is sumcient to produce cut-off at' tube 25 is just sufficient to overcome the negative bias of the anode 20 of tube I8 which, as

stated before, is equivalent to the sum of the volt- `ages e and e. The anode 20 of tube I8 is then -supplied with current and produces a potential drop across resistance 23, which potential drop is in turn applied to the control electrode of discharge tube 22. This voltage acts negatively on the controlelectrode andl tends to drive the'tube toward cut-a, thereby reducing the anode current in tube 22 and accordingly further reducing the negative bias on the modulator tubes 39 and tor tubes 39 and 40 Will be operated at maximum output and a maximum intensity of carrier fre'- quency will be present atv the output terminals l5 and 46. The output of the modulator tube is therefore varied in amplitude'between zero and a maximum in accordance with the intensity of the input signal as applied to the discharge tube I8.

Even though the output of the modulator tubes ranges from a minimum to a maximum in accordance with the rectified signalpresent at the output of the lter circuit, this relationship is not linear unless the parameters of the system are set for such operation. Normally, for carbon paper recording the tube 25 is set for high current and high gain sothat small signal increases abo-ve the threshold voltage value e give large lchanges in modulator output. At the point where this tube is biased to cut-off and tube 22 assumes control, the relationship changes since the tube 22 is set for low gain and low current with the 4result that the signal must increase a larger amount to produce a change in the output of the modulator tubes 39 and III).4 A curve of the overall operation of the system when vso adjusted is shown in Figure 4 and is marked R-low.

- is present when the resistance is made low may be accomplished so that the signal output from the compensating amplifier will change little during the operation of tube 25, the change increasing rapidly up to a maximum value during the subselquent operation of the tube 22. If it is desired that the compensating amplifier operate substantially linearly, then some average or intermediate value of the resistance R may be used as indicated by the curve marked R-medium in Figure 4.

It will be noticed that changesv in the value of the resistance R materially affect the response characteristics of the compensation amplifier as indicated by the curves shown in Figure 4, but it will also be noticed that the minimum and maximum amplitudes oi the carrier frequency in the 36 on the' resistance 34 are necessary to control the response of the compensation amplifier.

Such an amplifier,` as may be evident from an inspection of the curves shown in Figure ,4, may also be used as a compressor or expander. If the walue of the resistance R is low, then the amplifier will operate as a compressor and 'may be used, for instance, for making master phonography records. When the recording is played back through the amplifier with the value ofthe resistancev R set high, the amplifier will act as van expander so that the output' from the l amplifier willhave the same original tone and volume values'which were impressed upon the recording system. The circuit is particularly adaptable'for such use in View of the fact that it operates instantaneously on each wave rather 1 than on an average volume as in the case of the present phonograph expanders.

Referring now to Figure 2 wherein is shown a modified circuit for accomplishing the purposes set forth above. This circuit is similar to that shown and described in connection with Figure 1, but differs therefromfin that the modulator tubes are controlled in a different manner. In this figure the modulated input signal is applied to the transformer primary I2, the output of which is f..

connected to a double diode rectifier I4. A threshold double diode rectier tube 50 is provided having acathode 5I and a pair of anodes 52 and 53. The cathode 5i of the threshold rectifier 50 is connected to one terminal of the output from the rectifier tube I4 `and the anodes 52 and 53 are connected to the control electrodes of two separate electrony discharge tubes 55 land 56 respectively. The discharge tube 56 includes a cathode, a control electrode and an anode, whereu as the discharge tube 55 is of the pentode type including a suppressor grid and a screen grid.

For applying proper operating potentials to the electrodes of the various tubes, a source of current is applied to the terminals 3| and 32, the lat- F45 ter of which is connected to ground. A plurality of resistances 66, 6I and 62, which operate as a potentiometer, are connected inseries between the terminal 3l and ground. The anodes 51 and 58 of the discharge tubes 55 and 55 are connected to the potentiometer at a point between resistances 6I and 62,' resistances 64 and 65, respectively, being included in these connections. The control electrode of discharge tube 56 is connected tothe cathode thereof by way of a re- ,3

The anode 52 of the threshold rectifier 50 is connected to the lcontrol electrode of the discharge tube 55, which in turn is connected to the cathode of that tube by way of a resistor 68. The cathode of the tube 55 is connected to an adjustable point 10 along the resistance 60.

An oscillation generator including the tube 42 is provided for supplying a carrier frequency, which carrier frequencyis to be modulated in accordance with the intensity of the input signal.

This oscillator is-conventional in design and Vcor 75 responds to the oscillator' shown and described in connection with Figure 1. For coupling the carrier frequency oscillator to the modulator Vtubes 12 and 'I3 a transformer 'M vis provided.

'I'he modulator tubes l2 and 'i3 include a cathode, a control electrode, a screen or #2 grid, a suppresser or #S grid` and an anode. The cathodes are connected together and are in turn connected to a point between the resistances Gl and $2 of the potentiometer. In order that the carrier frequency may be applied to the tubes the ends of the secondary of transformer 'M are connected to the control electrodes of these tubes. The screen or #2 grids of the modulator tubes are connected together and are connected to the midpoint of the primary of the output transformer 44. These grids are also connected to the positive terminal 3| in order that they may be maintained positive with respect to their associated cathodes. 'Ihe anodes of the modulators 72 and 13 are connected to the end terminals of the primary of transformer M so that the modulated carrier current which appears in the anode circuits of these tubes may be utilized by connecting to the secondary of the output transformer.

The modulator tubes l and 13 are controlled by impressing signal or voltage variations upon the control electrodes and upon the suppressor or #3 grids. The midpoint of the secondary of transformer 'lll is therefore connected to the anode 57 of tube 55 so that the control electrodes vWill be affected by the current drawn by the tube y55 While the suppressor or #3 grids of the modulator tubes are connected together and are in turn connected to an adjustable point 'l5 along the anode resistor 85 of tube 5t. Current which is drawn by the tube 56 will therefore produce a potential drop along resistance 65 to control the potential of the suppressor electrodes of the modulator tubes 'l2 and '13, and the degree of this control may be adjusted by varying the position of the adjustable Contact 'l5 along the resistor 65.

In order to assist in the explanation of the operation of this system it will be assumed that the potential which appears between the adjustable contacts 63 and 59 along the resistor 5 be designated as the voltage b, whereas the voltage which appears between the adjustable contacts 63 and lli will be designated as Since the cathode of the rectier i4 is connected to the adjustable contact S3, it may be seen, therefore, that the anode 53 of the threshold rectier 50 is negative with respect to its cathode 5l by an amount equal to the voltage b. Similarly, the anode 52 of the threshold rectier 5o is negative with respect to its cathode 5l by an amount equal to the voltage b.

Modulated tone signals applied to the transformer I 2 are rectied by the rectiiier i4 and appear as a varying direct current voltage at the output of this tube. This direct current voltage is impressed upon. the cathode 5l of the threshold rectier 59 and vupon the resistance (it at the adjustable point E3. When the rectied voltage is less than the voltage b, no current will be passed by the threshold rectiiier 50, but when the rectied signal exceeds the voltage b, then ,current will be drawn by the anode 53 of the threshold rectier. This permits current to flow through the resistor 6l which tends to drive the discharge tube 5-5 toward cut-off. This reduction ,in the anode current of tube 56 reduces the negative bias of the suppressor or #3 grids oi the modulator tubes with the result that there is -an increase vin the carrier frequency passed by encarta these tubes. As the rectied input vsignal ln- `creases in potential, the discharge tube 56 is gradually driven to cut-oi, with the result that more and more carrier frequency is passed by the modulator tubes. The position of the adjustable contact 'lil is so set that at about the instant that the tube 55 is driven to complete cut-oil, the rectined signal input voltage is suiicient to overcome the voltage b', which results in anode current being drawn by the anode 52 of the threshold rectifier 5o. Current then begins to flow through the resistance iis which tends to drive the discharge tube 55 toward cut-01T. This reduces the current drawn by the discharge tube, 53 and ac cordingly the voltage applied to the control electrodes of the modulator tubes. 'l2 and T3 is changed tending to further increase the amount of carrier current passed by these tubes. Further increases in the rectilied signal input causes corresponding increases in the amount of carrier frequency passed by the modulator tubes 12 and t lf3 until a saturation or maximum limit is reached.

In View of the fact that the tubes 55 and 56 are different in structure and have different operating characteristics, the relationship of the current passed by the modulator tubes and the rectied potential as supplied by the rectifier I4 is diierent during the time periods that the tubes 55 and are controlling the operation of the tubes 'l2 and 13. The operation of the compensating amplifier is best shown by referring to Figure 5 wherein curves of the response of the amplifier are illustrated; No output from the amplifier is present until the input signal has reached a value corresponding to the potential b. At this point the tube 56 assumes control of the modulator tubes and the relationship between the input and output of the amplifier system may be indicated by that portion of the curve shown between the points b and b along the abscissae of the curve. At the point represented by the input voltage b', and for any greater voltage, the tube 55 assumes control of the modulator tubes l2 and i3 and the relationship or" this control is indicated by the remainder of the curve. over-all relationship is shown by the solid line curve of Figure 5. If it is desired that the compensating amplifier respond to a higher' or lower initial input voltage, then the adjustable point 53 may be moved along the resistance and the eilect oi such an adjustment is illustrated by the dotted curves shown in Figure 5. Such an adjustment, of course, determines the point at which the anode 53 of the threshold rectier 5l] is supplied with current, and also the point at which the modulator begins to be afected by the input signal. The over-all shape of the response curve may also be varied as indicated by the dotand-dash curves shown in the figure by controlling the amount oi the resistance contained in the anode circuit of the tube 55, which resistance is preferably made adjustable.

From the above it may be seen that Various shapes of response curves may be produced and .it may further be seen that changes in the overall response characteristics of the compensating amplifier do not affect the minimum or maximum outputfrorn the modulator tubes, but only the intermediate values thereof. The minimum output of the modulator may, however, be controlled by varying the position of the adjustable contact l along the resistance t5, so that varying degrecs of v.minimum carrier signal Yfrequency.may

-anodes 93 and v94, the latter of which is conbe'present in the output of' the amplifier' when no signal input is impressed upon the system;

The shape of the curves shown in Figure 5 corresponds vquite favorably with the curve necessary to produce proper halftones in facsimile recordings, such a response being capable ofA Will be noticed that demodulation of themodulated tone signal input is necessary, the output from the compensating amplifier being again modulated in order that the signals may be subsequentlytransmitted. There are instances, however, When such demodulation and subsequent modulation is objectionable and under such conditions a circuit such as that shown in Figure 3 may be used, the circuit operating as a compensating amplifier for a modulatedv signal which isnot demodulated or rectified. j

The system shown in Figure 3 comprises an input transformer 8l to the primary of which is connected the modulatedinput signal which may bederived from the scanning head of a facsimile transmitter. A pair of threshold tubes T-I and T-Z are used, each of which includes y a cathode and a pair of anodes. These tubes operate in push-pull, and to each one `of the tubes are v connected separate amplier tubes which have diierent operation characteristics.

For applying appropriate potentials to the electrodes of all of the tubes, a source of current is provided,I and is connected to` terminals 84 and 85, the latter of which `is vconnected to ground. A plurality of resistances 81, 88 and 89 are connected between the positive terminal 84 and ground so as to act asa potentiometer from Which different potentials may be derived.

TheV secondary of the input transformer 8| is provided with a center tap which is connected toan adjustable contact 9| on the resistance 88 while the ends of the secondary of the trans,- former are connected tothe cathodes of discharge tubes T-l and T--2.

As stated above, tube T-l includes a pair of nected to ground by Way of resistance 95, the resistance operating as a load resistance for the anode 94 and as a grid resistance for the discharge tube TY-3.` f Similarly, discharge tube 'T-Z includes a pair of anodes 96 and'97, the

latter of which is connected to ground by Way of resistance 98. In View of the fact that the anodes 94 and 91 of tubes T-I and T-2 are connectedto ground, While the cathodes of these tubes (that is, the mid-pointof the secondary of `transformer' 8l) are connected to the adjustable contact 9i on resistance 88,`it may be seen that the anodes 94 and 9T are negative with respect to their cathodes by an amount equal to the potential' of the adjustable contact 9|.

The anode 93 of tube T-l. is connected to a point between resistances 8l and 88 by resistance lill While the anode 96 of tube IL-2 is connected to the same point` by resistanceV |92. The anodesv 93 and 96 are also connected to the control electrodes of discharge tubes 'IL-5 and T-B respectively. The cathodes of tubes T-3 and T-4 arel connected toground, While the cathodes of tubes T--5 and T-B are connected to ajpointv between:resistanceszlVVA and 88. The anodes of tubes T-B and T-- are maintained positive With respect to Atheir cathodes .by connecting the anodes to the positive vterminal 84 through resistancesv |94 andk |95 respectively, While thev anodes of tubes T--3 and T4 aremaintained positive by reason of the fact that these anodes are connected to adjustable contacts along the resistanceslll and 195. resistances Illlland` |95 which is connected between the adjustable contacts and the"`positivel lThat lportion of the terminal 84 Willbe designated as Re as indicated in the drawings. y

The output from the compensating amplieris derived from the potential drops along kresistances |94 and m5 and'is applied to the primary of an 3 output transformer A|91 by a capacitycoupling through condensers |98. From the secondary of the transformerrll may, therefore, be derived the compensated andy amplified modulated signal Which was impressed .upon the input y transformer 8|.

For the purpose of explaining the operation of the system, the potential drop along resistance 81 will be designated asvvoltage ee While the potential drop along that portion of resistance 88 `between the adjustablecontact 9| and the junctionv of resistances 88 and 81, will be designated as et. The anodes 94 and 91 are, therefore, negative With respect to their cathodes by avvoltage equivalent to the sum of the voltages ec and et. Also, since the anodes 93 and 95 of the discharge tubes T-I and T-Zareconnected to a point betweenA resistances 8T and 88, these anodes WillA be negative with respect to the cathodes by an amount equal tc the voltage et. Inasmuch as this voltage is lessthanthe combined voltages et andes-,it is only necessary for the input signal to be of an amplitude equivalent to the voltage et to permit currentto flow to the anodes 93 and 96. Any signal in excess ofA the voltage et will thereforepermit passageof current through the resistancesy HJI andy H12 to control the operation ofl tubes T-5 and- T-6. No current `will rbe suppliedto the anodes l94 -and 9,1 until the input signal has at least reached an amount equivalent to the sum .of the voltages er andlec. At this time current Willbe supplied through the resistances 95 and 98 of the discharge tubes T-3 and T-4 so that a -control of these tubes will then be initiated. In view of the different characteristics oftubes T--3 and T`4 as ccmparedto tubes T-5 and T-6,vdiiferent overallamplication responses will be present depending'up'on the strength of the inputA signal. Furthermore, the combined elects of the tubes T-3 through T-B dependupon the value of the resistances Re, this resistance being common to the anodes of all of these tubes.

In Figures 6 and 7 are shown various curves indicating the response-of the system illustrated in Figure 3 and further illustrating the type -of response that may beaccomplished by usingsuch-v a system. 'I'he instant atv/hielo the anodes 93 and 94 are supplied with current depends upon the value of the voltage et, whichjmay be regulated lby moving the contact9l along the resistance 98. If this voltage is increased then the threshold `operating voltage'of the tubes T--l and T-2 is altered so that, as indicated-by Figures 6 and 7, the curves may be caused to be shifted.`

horizontally along the input axis of the curves. Accordingly, as the voltage et is increased, a greater initial input voltage is necessary before the tubes T- and 'IL-'4 lare controlled or are supplied with a control voltage from the anodes 93 and 96 of tubes T-l and T-2.

In Figure 6, the curves indicate the response possible when the gain or ampliiication of tubes T-5 and T- is greater than that of tubes T-S and T-lL In this case, if the resistance Re is low, then large changes in the output from the amplier will be present when small low voltage changes are applied to the system, whereas, if the resistance Re is made high, the compensating amplier will be less responsive to low voltage changes of input whereas largefchanges in the output of the amplifier will be present when higher voltage changes are impressed upon the` system.

In Figure 7 is shown a similar family of curves in which the gain of tubes T-E and T--tl is less than the gain of tubes T-3 and T-fL When this situation exists then the value of the resist--l ance Re will aiect the operation of the compensating amp-liner in a manner opposite to the effect which exists as explained above in connection with Figure 6. In other words, when the resistance Re is low, then the greatest response is attained when high voltage variations are present at the input Whereas when the resistance Re is high, the greatest percentage effect upon the output of the compensating amplifier is present during low voltage changes of input, that is, when the input voltage has increased just above the threshold value as determined by the value of the voltage et.

In Figures 6 and 7 it will be seen that the maximum and minimum signal output from the ampliiier remain substantially constant regardless of the value of the resistance Re or the value of the voltage et. For this reason changes in the response of the compensating amplier may be brought about by simply adjusting the value of the resistance Rc, and the instant at which the Acompensating amplifier begins to .operate may be determined solely by adjusting the position of the contact 9| along the resistance 88. When either the resistance Rc or the voltage et or both are adjusted no further adjustments to maintain the same minimum or maximum output from the compensating amplier are necessary.

It will also be noticed that the compensating amplier as shown in Figure 3 (which operates on a modulated wave Without demodulation of the wave) may be used as a compressoi or eX- pander as explained above in connection with Figure l, by simply varying the amount of the resistance Re;

From .the above description of the present invention, it may be seen that the over-all operation of a facsimile system maybe greatly improved by using any one of the described circuits. since with each it is possible to compensate for the non-linear operating characteristics of all of the elements in a facsimile transmitting and receiving system. In instances where the nonlinear response of the various elements of the complete system remain substantially constant the compensation amplier is preferably located on the facsimile transmitter, with the result that adequate compensation will have been inserted in the transmitted signal before it is received at the facsimile receiver. When such compensated sig `nals are received and utilized to produce a. facsimile recording, an over-all operation is produced which is desirable and which will result in the production .of facsimile recordings which compare very favorably with the original subject matter. In the transmission of photographs or subject matter wherein a'high degree of fidelity half-tcne transmission is; necessary, the compensating ampliers shown are particularly advantageous since by using such ampliers the halftones are more faithfully reproduced and resem-- ble the transmitted subject much more closely than has heretofore been possible.

Various modications of the systems shown and described herein are possible, without departing from the spirit and scope of the invention described herein, and it is desired that any and all of such modifications be considered Within the purview of the present invention except as limited by the hereinafter appended claims.

l claim:

l. An amplifying system comprising an elec-` tron discharge device having a. cathode and a pair of anodes, circuit means for normally maintaining said anodes negative with respect to said cathode by different amounts, means for applying single signal source of variable potential between. said anodes and said cathode whereby said annales will be supplied with current when the intensity of the signal potential variations exceed the negative bias ci the respective anodes, therznionic discharge means associated with each anode circuit, each of the thermionic discharge means having diiferent operating characteristics, means whereby said thermionic discharge means will respond to the presence of and amount of current in the associated anode circuit, and means for combining the outputs from said thermionic discharge means.

2. An amplifying system comprising a pair of unidirectional current paths, potential biasing means for opposing the flow of current through said paths the degree of the potential bias being different for the two paths, means for applying a single variable potential signal source to said paths whereby current may flow through said paths when their respective bias potentials are overcome by the signal potential, a pair of electron discharge devices each including at least a cathode, a control electrode and an anode, and eachhaving different operating characteristics, means for controlling said discharge devices in accordance with the presence and amount of current in the current paths, and a common output circuit connected to said anodes.

3. An amplifying system comprising a pair of unidirectional current paths, potential biasing means for opposing the now of current through said paths the degree of the potential bias being diiferent for the two paths, means for applying a vsingle variable potential signal source to said paths whereby current may flow through said paths when their respective bias potentials are overcome by the signal potential, a pair of electron discharge devices each including at least a cathode, a control electrode and an anode, and each having a diiierent control response, means for controlling said discharge devices in accordance with the presence and amount of current in the current paths, means for suppressing or accentuating the control response of said devices. and a common output circuit connected to said anodes.

4. An ampliiyingsystem comprising a plurality of unidirectional current paths, means for applying a diierent adjustable potential bias to each path to oppose current flow therethrough, means for applying a single variable potential signal source to said paths whereby current may flow through said paths when their respective bias potentials are overcome by the signalpotential,

an, electron discharge device associated with each f current path and responsive to the presence and amount of current passed by the associated path, each of said discharge devices having different operating characteristics, means for controlling the degree of response of said devices, and means for combining the outputs. from said discharge devices.

5. An amplifying system comprising a plurality of unidirectional current paths, means for apply# ing a different potential bias to each path to oppose current flow therethrough, means for applying a single variable potential signal source to said paths whereby current may flow through said paths when their respective bias potentials are overcome by the signal potential, an electron discharge device associated with each current path and responsive to the presence and amount of current passed by the associated path, each of said discharge devices having different operating characteristics, means for controlling the degree vof response of said devices, means for varying the potential bias applied to each current path, and means for combining the outputs from said discharge devices.

6. An amplifying system comprising a plurality of unidirectional current paths, means for applying a different potential bias to each path to oppose current now therethrough, means for applying asingle variable potential signal source to said paths whereby current may flow through said paths when their respective rbias potentials are overcome by the signal potential, an electron discharge device associated with each current path and responsive to the amount of current passed by the associated` current path, each of said devices including at least a cathode, a control electrode and an anode and each having a different response characteristic, a source of carrier frequency, means for modulating the carrier frequency, and output 'circuit means including the anode of each electron discharge device for controlling said modulation means.

7. An amplifying systemcomprising a plurality of kunidirectional current paths, means for applying a different potential bias to each path to oppose current flowtherethrough, means for applying a single variablepotential signal source to said paths whereby current may flow through said paths when their respective bias potentials are overcome by the signal potential, an electron discharge device associated with each current path and responsive to the amount of current passed by the associated current path, each of said devicesincluding atleast a cathode, a control electrode and an anode and 4each having a different response characteristic, a source of carmeans including the anode of each electron dis-l tubes. r

charge device for controlling said modulating means.

8. An amplifying system comprising a plurality of unidirectional current paths, means for applying a different potential bias to each vpath to oppose current oW therethrough, means for applying a variable potential signal source to said paths whereby current may flow through said paths when their respective bias potentials are overcome by the signal potential, an electron discharge device associated with each current path and responsive to the amount of current passed by the associated current path, each of said devices including an anode and each having a different response characteristic, a source of carrier frequency, means for modulating the carrier frequency, and circuit means including the anode of each electron discharge device for controlling said modulating means.

9. A compensating amplier system for a modulated signal comprising a pair of unidirectional current paths, means for applying a different potential bias to each path to oppose current flow therethrough, means for demodulating the signal to produce a potential variation, means for applyn ing the potential variation to said pathswhereby current may flow through said paths when the potential variations exceed their respective bias potentials, an electron discharge device associated with each current path and responsive to the amount of current in the associated path, each of said devices including an anode and each having a different response characteristic, a source of carrier frequency, means for modulating the carrier frequency, andcircuit means including the anodes of said discharge devices for controlling said modulating means.

l0. An amplifying system comprising a pair of thermionic discharge means each including a When the signaly potential variations exceed the negative bias of the respective anodes, a pairof thermionic discharge tubes connected respectively to said one anode in each discharge means, a second pair of thermionic discharge tubes connected respectively to said other anode in each discharge means, each of s'aid pairs of tubes hav-V ing different characteristics and each tube responsive to the presence and amount of current in the associated anode circuit, and meansfor combining the outputs from all of the discharge MAURICE AAn'rz'r. 

